Recently, a digital media and an electronic publishing business have been popularized and dramatically developed. Further, the rapid growth of the Internet makes communication easier and more extensive than before, while the advent of digital multimedia enables the creation and dissemination of products quickly via electronic means.
Since the digital data has no difference in quality between an original and a copy thereof, it is impossible to distinguish the original from the copy thereof. Accordingly, it is needed to protect Intellectual-property rights for audio, video, images and other documents and perform authentication, especially against an illegal copying, distribution and modification of the digital data.
To that end, information protecting methods, such as an encryption, a Firewall or the like, are conventionally employed. However, the methods have a drawback in that there is no solution to prevent users who are allowed to access data from illegally copying and modifying the data.
To solve these problems, in recent days a digital watermarking technique, which is known for its effectiveness to prevent the illegal copying of the digital data, is being widely used. The technique is used for hiding copyright information or user information so as to keep digital contents from being copied. To be specific, signals that are imperceptible under normal viewing conditions, i.e., specific data rows called as watermarks, are embedded into multimedia contents, e.g., audio, video, image and text contents, created by a copyrighter in order to protect copyrights thereon.
There have proposed several watermarking methods in a transform-domain Discrete Cosine Transform (DCT) based on a frequency domain. For example, in a method suggested by I. J. Cox, an N×N DCT is performed on an original image. Then, a real number sequence is added to upper DCT coefficients except DC components, wherein the number of the DCT coefficients is n, n being a positive integer. In another method, it is determined whether watermarks are embedded into an image or not, based on a Just Noticeable Difference (JND) value by using a Human Visual System (HVS). Then, the JND is included in image into which a watermark is embedded. Further, in a recent method, watermarks are invisibly embedded into the DC components in a DCT region.
However, as a highly efficient compression is required for image and video data, a great deal of development has been devoted to the image data compression using a wavelet transform. A compression standard, JPEG2000, is an image compression method based on the wavelet while a conventional JPEG standard is based on the DCT, so that various watermarking techniques employing the JPEG 2000 are being developed.
The watermark embedding method based on the wavelet transform is categorized into several methods. For instance, there is a method for embedding a watermark signal having a different length into every high frequency domain except a lowest frequency domain. Further, there is another method for embedding a watermark signal into a large coefficient. In most watermark embedding methods, however, it is considered that the HVS is sensitive to changes in low frequency components rather than those in high frequency components. Thus, after the wavelet and the frequency transform is performed, it is widely carried out to embed watermarks into remaining frequency components except the lowest frequency component, i.e., the DC component. However, in case a high compression rate generated by embedding the watermarks into the images is applied to the high and middle frequency components or in case an intentional attack occurs to remove the watermarks, it is impossible to detect the watermarks. As a result, a blind type watermarking method based on the wavelet, which is able to trade off distortion and robustness, has been required.